1. Field of the Invention
The present invention relates to apparatus and methods for providing image stabilization in high-performance optical systems.
2. Description of the Prior Art
Many optical systems used in manufacturing today are high-performance systems in which high-resolution images are formed. The images usually need to be imaged onto a light-sensitive medium, such as a photosensitive workpiece or a detector, with great precision.
An example of a high-performance optical system is a microlithographic projection lens. Such lenses are used in lithography systems, which are used to fabricate semiconductor devices such as microcircuits. Microlithographic projection lenses typically are required to resolve resolution-limit “critical dimension” features on a mask. Further, the images need to be accurately located on the wafer, i.e., to within nanometers of existing features.
In a lithography system, the position where a mask image (referred to as the “aerial image”) is formed in the image plane can vary due to refractive index variations of the gas (e.g., air) within the spaces between the lens elements for certain types of microlithographic lenses. The index variations can be caused by lens heating, which can arise from a number of sources such as electrical and mechanical elements within the lithography system (e.g., the wafer stage linear motors). If not properly dissipated, such heat can cause a negative thermal gradient across the housing of the lens, wherein the top of the lens is cooler than the bottom of the lens. This, in turn, can cause convective heat transfer within the lens, resulting in unstable air motion within the lens and thus variations in refractive index along the optical path. If the space between lens elements is relatively large (which is the case for many catoptric and catadioptric microlithographic lens designs), such refractive index variations can significantly alter the optical path of the light rays. This, in turn, can cause slight displacements in the image at the image plane, which can lead to alignment and/or stage positioning errors, which ultimately degrade the quality of the device being fabricated.